Setting Relative Humidity (RH) Parameters

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The conservation criteria which is set by the exhibit conservator establishes relative humidity parameters for objects in an exhibit.

What are Relative Humidity Parameters?

Museum professionals have understood the significant role of relative humidity in object preservation for decades. Based on research that showed different materials are best preserved at a specific relative humidity, a compromise environment for a mixed-media collection was promoted. The general recommendation frequently given over the past several decades for a moderate climatic zone was a fifty percent (50%) relative humidity plus or minus five percent (±5%). In recent years conservation scientists, such as Stefan Michalski1 and David Erhardt have re-examined the more rigid climatic recommendations of the past from the cost vs. benefit standpoint.

Our understanding of what constituents an acceptable museum environment, however, has continued to evolve.2

• Maintaining 50% relative humidity during winter months causes structural damage to some buildings.
• A 50% relative humidity is inappropriate for tropic and arid climates.
• Many collection materials are more tolerant of variations in humidity than was previously thought.
• No one RH is ideal, and any value chosen must be a compromise.3

What has remained evident is the important effect that relative humidity has on most types of collection materials. Although a humidity fluctuation of ±5% provides the most stable environment, many collection objects are not damaged by exposure to a wider range of humidity fluctuation. In these situations, a practical way of establishing an appropriate humidity range for an object is to identify both its preferred relative humidity-the target humidity-and the degree of fluctuation from this target that is safe for the object-the humidity range. This establishes the relative humidity parameter for that object. The parameters can be refined further by identifying how quickly the relative humidity can fluctuate within the range.

How are relative humidity parameters established for particular objects?

Selection of the target relative humidity and the acceptable range of fluctuation from the target depend on the moisture sensitivity and vulnerability of the objects; the museum's geographic location; and the exhibit circumstances. The following object categories and their allowable deviation in relative humidity are some of the considerations a conservator makes when establishing the conservation criteria for the exhibit.

CATEGORIES OF SENSITIVITY	SELECTED EXAMPLES	ALLOWABLE DEVIATION IN RELATIVE HUMIDITY
Category 1: High restriction	panel paintings elephant ivory lacquerware	±5% from target
Category 2: Mode rate restriction	furniture painted organics ethnographic objects	±10% from target
Category 3: Low restriction	wooden agricultural implements baskets	±15% from target
Category 4: Minimal restriction	stable decorative metals glass ceramics	±20% from target

Narrow ranges of relative humidity deviation from the target point are necessary for very sensitive objects. This category, called Category 1 in the table, requires the tightest relative humidity parameter and includes, for example, lacquerware, Japanese scrolls, panel paintings, unstable metals, and unstable glass. Category 4-the least sensitive to relative humidity allows the largest spread of relative humidity parameters and includes stable stone and stable glass.

Note that the central target relative humidity for objects in the same category of allowable deviation may differ from one another. The categories rank the reactivity of the material to relative humidity, but do not relate to any specific target point. Depending on the diversity of objects chosen for a display, there may be several relative humidity parameters for one exhibit. Microclimates created within exhibit cases are very effective at meeting different relative humidity criteria for the enclosed objects. A practical approach is grouping together collection objects with similar relative humidity parameters.

The following examples illustrate how the idea of relative humidity parameters are used.

Example 1: A panel painting considered to be highly sensitive to relative humidity fluctuations and acclimated to a relative humidity of 50% will require a tight parameter in relative humidity control. The conservation criteria will call for a Category 1: High Restriction environment based on a target RH of 50%. Due to its extreme sensitivity, the conservation criteria might also limit the amount of fluctuation to less than 5% over any twenty-four hours.

Example 2: An archeological steel sword that has been treated for active corrosion but appears to be stable is not considered to be particularly sensitive to relative humidity. It will be better preserved at a lower relative humidity. The conservation criteria might set a Category 4: Minimal Restriction environment be maintained around target humidity of 30%. This means that the relative humidity can fluctuate from 10% to 50%.

1 Given the typical mixed historic collection and the possible extremes of climate, the most benefit is achieved by staying within the range of 50% ±20% RH and 68° ±32°F, and doing so reliably over a hundred year peri- od. Cutting these fluctuations in half brings real but rapidly diminishing benefits. A consensus is emerging, both in CCI's experience and that of others, that staying within ±10% RH is reasonable to expect in large- scale building mechanical systems. Cutting them in half again (±5% RH) brings very small improvement. Stefan Michalski, Environmental Guidelines: Defining Norms for Large and Varied Collections, AIC Workshop Papers, Norfolk VA, June, 1996, p.28-33.

2 The approach outlined in the TechNote is consistent with conclusions reported from a seminar held in September 1997 to develop guidelines for the selection of appropriate environmental parameters for individual collections. The Conservation Analytical Laboratory (Smithsonian Institution), together with the National Center for Preservation, Technology and Training (National Park Service) organized the seminar to review recent research in the field. Excerpts from these Environmental Guidelines were published in the NCPTT Newsletter, Issue 21, November 1997.

3,4 RH Stability Zones Table: The ranges of relative humidity suggested by consideration of various factors, David Erhardt and Marion Mecklenburg, Relative Humidity Re-examined, Preventive Conservation: Practice and Theory and Research. Preprints of the Contributions to the Ottawa Congress, London: IIC, 1994, p.32-38.